Recent advances in metallic transition metal dichalcogenides as electrocatalysts for hydrogen evolution reaction

Summary Layered metallic transition metal dichalcogenides (MTMDs) exhibit distinctive electrical and catalytic properties to drive basal plane activity, and, therefore, they have emerged as promising alternative electrocatalysts for sustainable hydrogen evolution reactions (HERs). A key challenge for realizing MTMDs-based electrocatalysts is the controllable and scalable synthesis of high-quality MTMDs and the development of engineering strategies that allow tuning their electronic structures. However, the lack of a method for the direct synthesis of MTMDs retaining the structural stability limits optimizing the structural design for the next generation of robust electrocatalysts. In this review, we highlight recent advances in the synthesis of MTMDs comprising groups VB and VIB and various routes for structural engineering to enhance the HER catalytic performance. Furthermore, we provide insight into the potential future directions and the development of MTMDs with high durability as electrocatalysts to generate green hydrogen through water-splitting technology.


INTRODUCTION
The development of renewable energy technologies has become indispensable given the increasing energy issues related to energy security, environmental pollution, and sustainable economy (Dresselhaus and Thomas, 2001). Hydrogen, which has diverse advantages such as high energy density (142MJkg À1 ), safety, and recyclability, has emerged as a promising energy carrier for achieving zero carbon emissions. The electrochemical reaction from water has also gained attention as a sustainable method for generating green hydrogen; however, the ultimate potential in hydrogen evolution reaction (HER) is yet to be accomplished owing to the use of scarce and expensive precious metals as electrocatalysts (Turner, 1999). Several attempts have been made to solve this problem by lowering the Pt content of the electrocatalysts while maintaining high HER activity (Cheng et al., 2016;Lin et al., 2017). Because of the low utilization efficiency, not all Pt atoms in a typical Pt-based catalyst are active, and Pt single atom-based catalysts tend to agglomerate during catalytic processes, leading to a decrease in the HER activity . Thus, there is a need for designing novel low-cost, earth-abundant electrocatalyst based on non-precious metals possessing high HER activity and long-term stability.
Various transition metal-based materials, including chalcogenides, phosphides, nitrides, carbides, and oxides, have been extensively researched and predicted to be high-performing HER catalysts (Bhat et al., 2021;Jaramillo et al., 2007;Popczun et al., 2013;Wazir et al., 2022). In particular, layered transition metal dichalcogenides (TMDs) have been recognized as excellent substitutes for Pt-based groups owing to their outstanding chemical stability and theory-guided discovery, which indicate their high HER activity (Hinnemann et al., 2005). Hinnemann et al. revealed that the Mo(10-10) edge in MoS 2 have a Gibbs-free energy of hydrogen adsorption (DG H* ) of 0.08 eV, which indicates near optimal binding energies of reaction (Hinnemann et al., 2005). Jaramillo et al. first demonstrated by electrocatalytic measurements that the catalytic efficiency of 2H-MoS 2 exhibits a strong linear dependence on the number of Mo edge sites (Jaramillo et al., 2007). Owing to the inactive MoS 2 basal plane with a DG H* of 1.82 eV, the researchers have studied one strategy for exposing several edges on the restricted area (Kibsgaard et al., 2012;Ye et al., 2016). However, they faced limitations such as low conductivity and physical issues related to catalyst overloads that cause a decrease in charge and mass transport (Benck et al., 2014).  nature, whereas the partially occupied d-orbitals cause the metallic behavior of TMDs. Under ambient conditions, two electrons in group VIB TMDs tend to fill the d z 2 level (2H) with priority over t 2g levels (1T) because the required energy is lower. They prefer the thermodynamically stable semiconducting-2H phase to the metastable metallic-1T phase, except for WTe 2 , which exists as the T d phase. As shown in Figure 3C, the 1T 0 phase possesses even lower ground-state energy without mechanical stress compared to that of the 1T phase (Duerloo et al., 2014;Hernandez Ruiz et al., 2022;Zhang et al., 2016). Therefore, structural distortions by mechanical stress or electron injection have attracted extensive attention for improving the structural stability of group VIB MTMDs. In the case of group VB TMDs, both the 2H phase and 1T are stable because the energy difference between the 2H and 1T phases is less than 0.1 eV. Their d-orbitals are partially filled by the additional electron regardless of the crystal field, and therefore, both the 2H and 1T-group VB TMDs are always metallic. Recent studies on group VB MTMDs revealed extended four structures including 2H (Jeong et al., 2012), 3R , 1T (Fu et al., 2016), and 1T' (Li et al., 2018a).
After a long time, the oxidation rate of TMDs can be accelerated by reaction with water and oxygen, starting from the edge or the defects, regardless of the phase and chalcogen (Voiry et al., 2013a). Of interest, the spontaneous initial oxidation of MTMDs (e.g., 1T-MoS 2 and 1T-TaS 2 ) can passivate the edge and protect the material against more severe oxidative degradation (Martincová et al., 2020). The adsorbed oxygen atoms eventually form a SO 2 group binding to one of the edge S atoms, resulting in a thermodynamically favored state with an intermediate S-O-MO structure (Pet} o et al., 2018). This oxidation is not harmful to HER catalysis in 1T-MoS 2 but may adversely affect other intriguing functionalities and phenomena reported for MTMDs (Figures 3D and 3E). The structural stability of TMDs is well maintained in acidic and alkaline solutions, both of which are HER-driven environments. Wang et al. demonstrated that the dissolution ratio of MoS 2 in acid is three orders of magnitude higher than that of TMP (e.g., CoP and MoP) but comparable to that of Pt . Although the HER performance of MTMDs in alkaline solution has not yet been well explored, they are reported to have poorer activity in the alkaline medium than in an acidic medium. Moreover, several studies have reported similar results regarding the stability in acidic media as well .

Correlation between electronic structure and hydrogen production
In general, the HER is a multi-step reaction that includes adsorption, reduction, and desorption steps, which are highly dependent on the intrinsic chemical and electronic properties of the electrode surface as well as the electrolyte (Table 1). Most studies have reported that the performance of electrocatalysts in alkaline solutions is inferior to that in acidic solutions (Li et al., 2017a). This is because in alkaline solutions, additional energy is required in the Volmer step to dissociate the water molecules, whereas in the acidic medium, the electrolyte releases protons from the hydronium cation (H 3 O + ). Three associated descriptors were used to evaluate the ease with which a catalyst initiates the reaction: water adsorption energy (E ad ), activation energy of water dissociation (E ac ), and DG H* . Among the three parameters, DG H* is the most commonly known, and it indicates the binding strength of H* on the catalyst surface in both acidic and alkaline solutions. According to the Sabatier principle, optimal electrocatalysts for HER can have moderate binding energies of hydrogen adsorption. The DG H* of edge in 2H-MoS 2 -one of the TMDs-based electrocatalysts-is located below the precious metals, which indicates an enormous potential as an electrocatalyst with high activity, as shown in the volcano plot ( Figure 3F) (Jaramillo et al., 2007). Recently, it was demonstrated that the basal plane of 1T-MoS 2 (Voiry et al., 2013a(Voiry et al., , 2016 and S vacancies with the localized metallic states in 2H-MoS 2 (Hinnemann et al., 2005) also allow for increasing HER activity. Figure 3G reveals that the energy at or near the lowest unoccupied states (ε LUS ) has a linear relationship with the corresponding surface adsorption energy (E a ) . The polymorphism of TMDs is a key origin to achieve a high density of electronic states at the Fermi level, and this promotes the electrode kinetics for HER Wang et al., 2017b). Considering the HER mechanism, various approaches have been made to develop outstanding electrocatalysts that have high intrinsic activity and a large active surface area, in addition to allowing fast charge transfer and exhibiting prolonged electrochemical stability. Among them,  (Martincová et al., 2020). Copyright 2020 IOP Publishing Ltd. (F) Volcano plot of the exchange current density (i 0 ) as a function of the DG H* for pure metals and MoS 2 . Reproduced with permission from (Jaramillo et al., 2007). Copyright 2007, The American Association for the Advancement of Science. (G) Correlation between the ε LUS descriptor and surface adsorption energy (E a ). Reproduced with permission from . Copyright 2017, Nature Publishing Group. Plot for the DG H* of the basal plane in (H) semiconducting and (I) metallic monolayer TMDs vs. DG HX . Reproduced with permission from (Tsai et al., 2015). Copyright 2015 Elsevier B.V. iScience Review group VB TMDs possess ε LUS (z -6 eV) that correspond to near-zero E a values. Moreover, the metallic conductivity of their stable polymorphs and the presence of active sites in their basal plane enable improved catalytic activity and faster reaction kinetics. Therefore, they show higher potential as efficient and durable electrocatalysts than do group VIB STMDs. Similarly, Figures 3H and 3I indicate the plot of the DG H* as a function of the Gibbs-free energy of HX adsorption (DG HX* ) at the basal planes of the semiconducting and metallic TMDs (Tsai et al., 2015); thus, a more stable electrocatalyst requires higher H-X binding. This is because the X atoms in TMDs can withdraw electrons from the transition metals owing to their higher electronegativity, and X can act as the active site to stabilize the reaction intermediates. Figures 3G-3I theoretically demonstrate that the metallic basal plane of the TMD plays a more critical role in modulating the HER activity than do the structure and composition. In this review, we focus on MTMDs-based electrocatalysts.

Synthesis of MTMDs
Undoubtedly, the controllable synthesis of MTMDs with a high crystalline quality, thickness uniformity, large domain size, and continuity is critical not only to manipulate electronic structure but also extensively investigate catalytic properties and unique physical properties such as charge density wave (CDW) (Xi et al., 2015), superconductivity (Navarro-Moratalla et al., 2016), and ferromagnetism    phase. In this section, we will provide an overview of the synthesis strategies to achieve high-quality MTMDs-based electrocatalysts.

Top-down approach
The top-down method is a leading route to obtain low-dimensional, supreme quality TMDs single crystal. Most investigated novel physical properties of TMDs are demonstrated in exfoliated flakes from bulk crystals (Geim and Grigorieva, 2013). Chemical vapor transport (CVT) is typical approach for the single crystal growth of TMDs in the bulk foam ( Figure 4A) . MX 2 structures can be produced in various compounds by reacting transition metals and chalcogens with a selected mineralizer, as shown in the inset of Figure 4A (Lv et al., 2017). Thermodynamically stable group VB MTMDs can be easily obtained by mechanical/chemical exfoliation from bulk group VB MTMDs. Yan et al. produced metallic multilayered VSe 2 nanosheets by Scotch-tapebased mechanical exfoliation and the evaluated tunability of HER performance by applying a back gate voltage ( Figure 4B) . Owing to the limited sample size and poor production rate, this approach is incompatible with a large-area synthesis of MTMDs. Instead, liquid phase exfoliation (LPE) has been advanced for the scalable production of MTMD films (Lin et al., 2018). In this process, the interlayer spacing of MTMDs is expanded by the insertion of external ions such as Li-ion and ammonium ion. In addition, the expanded MTMDs are laminated using an external mechanical driving force. As shown in Figure 4C, Najafi et al. obtained few-layer H-TaS 2 and H-TaSe 2 flakes with a lateral size of 10-450 nm by LPE via 2-propanol (Najafi et al., 2020).
The high-quality H-TaS 2 flakes formed by batch production act as outstanding electrocatalysts for HER.
The synthesis of metastable group VIB TMDs in the bulk requires higher formation energy as compared to the stable 2H-group VIB TMDs. Phase transition via external force (charge transfer (Kang et al., 2014), electric field (Shang et al., 2019), and mechanical stress (Duerloo et al., 2014)) is a major approach to stabilize a metastable phase. In 2013, Voiry et al. reported that exfoliated monolayer WS 2 with a high concentration of metallic 1Tedges using a Li-intercalated LPE method served as an efficient electrocatalyst for hydrogen evolution ( Figure 4D) (Voiry et al., 2013b). In the LPE process, the as-prepared 2H-WS 2 powder was Li intercalated to form Li x WS 2 . Although there is a sufficiently large energy barrier in the 2H to 1T phase transition, their energy barrier is unsubtly lowered with the assistance of Li + intercalation (Xia et al., 2017); it is attributed to modulating the electron injection from a semiconducting to a metallic one via Li-intercalation. Therefore, they obtained the as-exfoliated WS 2 nanosheets with zigzag-like local distorted lattice configuration by strain as shown in the highangle annular dark-field scanning transmission electron microscope (HAADF-STEM) image (the middle of Figure 4D).

Bottom-up synthesis
The control of the process and crystal quality of obtained samples are poor although the LPE method provides feasibility for the batch production of MTMDs. Many researchers are investigating strategies to develop a scalable production of high-quality MTMDs. Bottom-up synthesis is the most notable way to increase their potential application and practical utilization. Recent studies revealed that some metastable group VIB TMDs may be directly synthesized via solution-based reaction and chemical vaporization.
However, the reliable method used to attain group VIB MTMDs is in the early stages of the study. Furthermore, the synthesis of group VB MTMDs is limited because the number of available M precursors in group VB is small and most of them have high melting points (Table 2). In this section, we aim to provide the current status of the bottom-up synthesis of MTMDs.

Solution-based method
Post-treated group VIB MTMDs using a conventional method such as liquid exfoliation (Voiry et al., 2013b), electron-beam irradiation (Cho et al., 2015), mechanical strains (Duerloo et al., 2014), and plasmonic hot electrons (Kang et al., 2014) are transformed readily into the stable 2H-phase via intermediates and oxidation. A recent study reported that ambient stable 1T-MoS 2 and 1T-WS 2 for more than 1 year were synthesized using the facile hydrothermal method under a magnetic field that optimizes the kinetics reaction ( Figure 5A) . The hydrothermal method refers to a heterogeneous reaction that depends on solubility in water or an organic solvent in a sealed steel container with Teflon liners. The reaction occurs in a low-temperature range of 100-200 C under the pressure generated by the container. However, Ding et al. applied a magnetic field that can transfer high energy on an atomic scale of the substance in addition to a general hydrothermal method. The HAADF-STEM images of the hydrothermally synthesized MoS 2 at 9 T and 0 T illustrate the difference in the atomic configuration ( Figures 5B-5E). Figures 5B and 5C show the 1T-phase as judged from the intensity profile, which indicates that S atoms are dispersed uniformly around the Mo atoms. In contrast, the intensity variations of 2H-phase are detected, wherein the two duplicating sulfur atoms amplify the signal along the electron beam direction ( Figures 5D and 5E); these results indicate that the stable 1T-(Mo, W)S 2 originates from the enhanced kinetics. Further, Zhou et al. presented the formation of 1T-MoSe 2 nanosheets via interaction with charged reaction by-product . Figure 5F depicts the synthetic procedure of the expanded 1T-rich MoSe 2 nanosheet using ethylenediamine (NH 2 C 2 H 4 NH 2 ) that plays a critical role during hydrothermal reaction. NH 2 C 2 H 4 NH 2 is decomposed NH 4 + , and then generated NH 4 + intercalates into MoSe 2 . As shown in Figures 5G and 5H, NH 4 + -intercalated 1T-rich MoSe 2 nanosheets were obtained because the 1T structure is stabilized by charge transfer from NH 4 + to MoSe 2 . iScience Review Compared to group VIB MTMDs, the synthesis of thermodynamically stable group VB MTMDs using a solution-based method is relatively easy; however, few studies have been performed except for VX 2 because of the low solubility of the group VB precursor in water and organic solvents (Table 1). Figure 5I displays the grown 1T-VS 2 nanoplates by hydrothermally reacting Na 3 VO 4 $10H 2 O that has a high solubility in water (221.7gL À1 at room temperature as summarized in Table 1  The representative Scanning electron microscopy (SEM) images in Figures 5J-5L indicate that 1T-VS 2 nanoplates with $30 nm in thickness and $800 nm lateral dimension were grown uniformly, and they fully cover the skeletons of the carbon paper. They suggested that the low cost and scalable solution synthesis of VS 2 catalyst can enable a promising electrocatalysts for large-scale hydrogen production.

Chemical vapor deposition method
CVD has been widely used to perform synthesis of high-quality TMDs. The morphology, thickness, and defect in CVD-grown TMDs can be elaborately controlled because of the broad tunability of their substrates and growth parameter (including precursor, growth temperature, working pressure, growth time and carrier gas) (Kim et al., 2019; Lee et al., 2020). Group VIB MTMDs have achieved only MX 2 materials with a relatively stable 1T 0 structure. In 2018, Liu et al. initially reported the direct synthesis of 1T 0 -MoS 2 monolayers with high purity and superior quality using a one-step CVD process (Liu et al., 2018b). Figure 6A exhibits that the effect of intermediate K x MoS 2 formed during the process on the phase stability of the MoS 2 monolayers using the density functional theory (DFT) calculation. As shown in the reaction formula ( Figure 6A), the injected H 2 gas plays a critical role in building a reductive atmosphere. Therefore, 1T 0 -MoS 2 triangular thin flakes are synthesized successfully in the mixture of H 2 and Ar ( Figure 6B). Figure 6C shows five Raman peaks demonstrating a 1T 0 -MoS 2 nature. In pure Ar, high-quality triangular 2H-MoS 2 monolayers ( Figure 6D) are obtained, as revealed by the Raman spectra ( Figure 6E). Kwak et al. reported a novel scalable process to obtain single-crystalline MTe 2 (M: W, Mo) nanobelts on the desirable substrates at low temperature (% 500 C) and short growth time (% 10 min) (Kwak et al., 2018). The production of a high-quality stoichiometric MTe 2 layer with spatial homogeneity is limited because of the low activity of tellurium during process and structural instability by oxidation under ambient environment (Naylor et al., 2016). Eutectic alloy (e.g., Cu x Te y ) was employed as a Te precursor to synthesize high-quality MTe 2 nanobelts, as indicated in Figure 6I. Single crystalline T d -WTe 2 nanobelts on a 4 in. SiO 2 /Si were synthesized using this novel approach. The structural and surface analyses ( Figures 6K-6M) indicate that the phase, composition, and dimensionality of all MTe 2 crystals are manipulated using the control process based on the growth parameter (such as temperature and time). Furthermore, the MTe 2 nanobelts can be directly synthesized onto a targeted surface. Then, Sim et al. demonstrated that eutectic alloy-assisted growth is a promising method for the scalable production of MTMDs-based electrocatalysts, which presents the structural controlled W-based TMDs electrocatalysts with efficient HER activity via this method (Sim et al., 2021).
Recently, the CVD method revealed considerable potential in the growth of group VB MTMD with a large domain size and a controllable phase Shi et al., 2017). However, oxides of groups VB and VIB showed relatively high melting points, which inhibit the decomposition of M precursors through thermal annealing (Table 1). There are two methods to overcome this issue: (1) An alkali halide-assisted method and (2) the use of transition metal chlorides with lower melting point as an M precursor. In 2018, Zhou et al. discovered that molten salt can widely decrease the melting points of diverse transition metal oxides. The growth mechanism of molten-salted CVD method is illustrated in Figure 7A (Zhou et al., 2018a). As a representative example, the SEM images in Figure 7A show a comparison of the observed Nb nucleus with and without salt, which reveals a strong mass flux of the M precursor improved by the salt. Some metal oxides can combine with salt to generate metal oxychlorides; these decompose at a suitable temperature and enable the formation of thin 2D group VB MTMDs nanoflakes ( Figure 7B). The result of thermogravimetry and differential scanning calorimetry (TG-DSC) as shown in Figure 7C suggests that the decomposed temperature of salts mixed with all transition metal oxides reduced within the temperature window from 600 C to 850 C. Further, the mechanism of the molten-salted CVD process was widely adopted in several studies on the growth of TMDs Wang et al., 2017a).
Ji et al. reported that VS 2 nanosheets were successfully grown using the typical CVD setup using transition metal chloride VCl 3 (Ji et al., 2017). Figure 7D shows the facile CVD process to grow VS 2 nanosheets under a mixed Ar/H 2 gas flow with various substrates. A typical optical microscopy (OM) image in Figure 7E shows thin VS 2 nanosheets with an edge length of $40 mm. Of interest, the morphology of VS 2 nanosheets is controlled using the H 2 flow rate. The evolution of the average edge length as a function of the H 2 flow rate is plotted in Figure 7F, which shows the tunability of the dimension. Their application as electrodes of the electronic device and the energy conversion system is proved using high-dense VS 2 nanosheets ( Figure 7G). In a similar method using TaCl 5 , Shi et al. fabricated thickness-tunable 2H-TaS 2 flakes and large-area films on Au foil; then, they evaluated the feasibility as electrocatalysts for HER . The obtained TaS 2 -based electrocatalysts showed high HER performance, which proves that the CVD iScience Review method serves as a strategy for producing an efficient electrocatalyst. Of interest, Huan et al. reported that NaCl powder as a growth template is an effective electrocatalysts to trigger the scalable synthesis of group VB MTMDs in CVD (Huan et al., 2019). The synthesis process and 3D structure of TaS 2 on NaCl are indicated in Figure 7H. A considerable amount of large TaS 2 nanosheets are discovered vertically on the corners and curved surfaces of the micron-sized multilevel NaCl crystals, as indicated by the SEM images in Figures 7I  and 7J. After the CVD process, the NaCl crystal powder displayed a visible color change from white (before) to gray (after) ( Figure 7K). A purified TaS 2 nanosheet was achieved easily by dissolving the NaCl crystals in deionized water and then filtrating the solution through a filter membrane ( Figure 7L). As indicated in Figure 7M, the filtrated TaS 2 was directly dispersed in target solvents for subsequent characterization. The fabricated TaS 2 electrocatalysts via this method exhibited outstanding HER activity. Comprehensive research covering the scalable production, green transfer, and energy-related application of high-quality MTMDs was presented by advancing a novel NaCl template-mediated growth approach.

Engineering of MTMDs-based electrocatalysts for HER
Many researchers have attempted to develop commercially practical MTMD-based electrocatalysts because of the discovery of 1T-MoS 2 as the electrocatalysts (Lukowski et al., 2013). The catalytic active

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site of MTMDs is present at both edges and basal planes, and these materials exhibit superior electrical conductivity and optimal DG H* close to zero (Pan, 2014). Group VIB MTMDs have been initially devoted to evolving their HER activities via tuning electronic and structural features Sokolikova and Mattevi, 2020). Although group VIB MTMDs have been tremendously explored toward distinguished electrochemical catalysts, critical restrictions such as thermodynamic instability and shortage of controllable synthetic method remain (Yu et al., 2018). Furthermore, structural engineering techniques using group VB MTMDs with structural durability, which include edge engineering (Guo et al., 2020), defect engineering , and interfacial engineering (Gnanasekar et al., 2019), have recently been introduced as another key strategy to accelerate hydrogen production. Under this section, we closely describe the fundamental catalytic properties of MTMDs consisting of group VB and group VIB and focus on comprehending the influence of structural modification on the HER catalytic activities in the MTMDs.

Intrinsic catalytic
The enhancement of electrocatalytic performance depends on the specific active site of materials that lead to hydrogen adsorption and desorption. Understanding the fundamental differences between natural active sites is critical for studying the mechanism and subsequent designing of catalysts that can accelerate HER activity Jiao et al., 2015). Increasing the intrinsic catalytic activity directly results in improved electrode performance in a manner that mitigates the transport issues arising from the higher catalyst loading (Benck et al., 2014). Thus, we investigated the intrinsic activity of the catalyst on a per-site basis.

Group VIB MTMDs
Thermodynamically stable group VIB MTMDs indicate inhibited electrochemical capabilities because of their semiconducting nature. For enhancing the original catalytic activities of group VIB MTMDs, the phase transition from the semiconducting 2H to the metallic 1T phase was demonstrated as the facile strategy (Hu et al., 2017a;Tang and Jiang, 2016). Combined theoretical and experimental methodologies reveal that the conversion of 2H to 1T MoS 2 enhances HER catalytic performance (Lukowski et al., 2013;Wang et al., 2013). Voiry et al. proved that the highly concentrated conducting 1T phase of exfoliated MoS 2 nanosheets induced superior HER activity via experiments (Voiry et al., 2013a). Edge sites of 2H and 1T-MoS 2 were partially oxidized by soaking in aqueous solutions with oxygen saturation and performing cycles, which were confirmed by TEM images (Figure 8A), to investigate the catalytic mechanism. The edge-oxidized 2H-MoS 2 presented a more decreased activity than 2H-MoS 2 , whereas the performance of 1T-MoS 2 and edge-oxidized 1T-MoS 2 is similar ( Figure 8B). This result indicates that the oxidation of 1T-MoS 2 is unaffected by the catalytic properties. McGlynn et al. reported that the changes of electronic structure in 1T 0 -MoTe 2 dramatically improve catalytic activity when operating HER at the cathodic bias (McGlynn et al., 2019). As shown in Figure 8C, the overpotential (h) of 1T 0 -MoTe 2 reduced from 320 mV to 178mVat the current density of 10mVcm À2 after only 100 cycles. Based on the theoretical calculation, HER activity can be enhanced by changes in the electronic structure caused by electron doping under an applied potential while maintaining 1T 0 -MoTe 2 ( Figure 8D).
On-chip devices have been used to investigate atomically intrinsic active sites by exposing selective surfaces and to facilitate catalytic activity by enhancing the electrical coupling of MTMDs Zhang et al., 2017a). The edge contact technology using micro-electrochemical cell is essential to minimize contact resistance and confirm inherent catalytic properties (Zhou et al., 2018c). As shown in Figure 8E, a microelectrochemical cell composed of monolayer MoS 2 was manufactured initially (Voiry et al., 2016). The HER activity according to the active site was selectively evaluated by covering or exposing the edge of MoS 2 using e-beam lithography ( Figures 8F and 8G). Figure 8H shows that HER activity at the basal plane of MoS 2 irrespective of the phases is improved by decreasing the contact resistance and increasing the charge transfer, which demonstrates a mutual relationship between catalytic properties and electrical coupling.

Group VB MTMDs
Although the catalytic activity of semiconducting group VIB TMDs is revealed only on edge sties, group VB MTMDs comprising diverse crystal structures exhibit highly active sites on both edges and basal planes based on theoretical DFT results for DG H* in Figure 9A  iScience Review A cm À2 from the Tafel slope than those of 2H-MoS 2 (479 mV and 6.3 3 10 À10 A cm À2 ) ( Figures 9C and 9D). The basal plane of group VB MTMDs, especially NbS 2 , was confirmed as the active site and was considered a promising HER catalysis beyond group VIB MTMDs.
Some group VB MTMDs catalysts such as TaS 2 and NbS 2 have the unusual intrinsic ability of self-optimum performance based on highly enhanced active basal plane sites when negative potentials are continuously applied for HER activation Zeng et al., 2014). In addition, Liu et al. addressed the HER kinetics of group VB MTMDs for H-TaS 2 and H-NbS 2 multilayer platelets prepared by CVD growth . Compared to different MTMDs-based catalysts, H-TaS 2 requires a relatively low overpotential of 60 mV to yield a current density of 10mAcm À2 ( Figure 9E). H-TaS 2 needs to repeat 5,000 cycles for optimizing catalytic activity to achieve this HER performance ( Figure 9F). Atomic force microscopy (AFM) profiling as shown in Figures 9G and 9H displays a thinner thickness of optimized H-TaS 2 that ranges from about 100 nm to 150 nm than that of H-TaS 2 measured before cycling (approximately 300 nm to 400 nm); this implies that these morphological changes acquired during electrochemical reaction result in enhanced catalytic activity. Charge-transfer resistances are obtained by the electrochemical impedance spectra (EIS) decreased following the repeated number of cycles until 5,000 cycles, which denotes shorter electron-transfer pathways. The calculated double-layer capacitance by EIS analysis boosted as cycling measurement is repeated, and this implies an increase in the active surface area ( Figure 9I). Following these beneficial features, they proposed that the generated H 2 bubbles on the basal plane between group VB MTMDs interlayers can be trapped; the trapped H 2 gas moves to escape leading to exfoliate or perforate layers ( Figure 9J). The self-optimizing HER activity of the pre-mentioned group VB MTMDs (H-TaS 2 and H-NbS 2 ) improved the charge transfer and accessibility of active sites induced by morphological changes. This offers a promising platform to apply scalable electrochemical devices that can surpass traditional TMDs.  . The facile process for creating defects that transform the electronic structure has been developed to increase edge sites (Hong et al., 2015;Li et al., 2018c). Post-treatments performed after the growth of materials using strain (Li et al., 2016b;Voiry et al., 2013b), thermal annealing (Najafi et al., 2020;Yin et al. 2016), and plasma (Ye et al., 2016) provide controllable defect density and improve active edge sites. Figure 10A shows low-energy oxygen (O 2 ) plasma processing induced a tremendous density of atomic-scale pore defects in the basal plane of metallic TaS 2 sheets (Li et al., 2016a). The result of STEM analysis ( Figures 10B-10E) shows the number of pores controlled depending on the plasma treatment time from 0 to 15 min. Treated TaS 2 electrocatalysts with an optimal defect concentration for 10 min showed the lowest onset potential of 200 mV and charge transfer resistance ( Figures 10F and 10G). The post-treatment using O 2 plasma that generates a large number of atomic-scale pores was demonstrated to increase the exposed edge of TaS 2 in the basal plane, which enhances HER activity. In addition, Najafi et al. presented thermal annealing under an H 2 -rich atmosphere as post-treatment to control the defect of metallic TaS 2 (Najafi et al., 2020). The TaS 2 films obtained through the filtration of the colloidal solution comprising 2H-TaS 2 flakes were annealed at 600 C under Ar/H 2 . Hydrogen plays a major role in TaS 2 etching while generating H 2 S gas ( Figure 10H). The annealed 2H-TaS 2 films exhibited boosted porosity for promoting the ion adsorption rate and increased quantity of edge sites because of diminishing sulfur content ($14%) ( Figures 10I-10K). These properties of the 2H-TaS 2 electrode significantly influence catalytic activity in both acidic and basic electrolytes. Compared with the conventionally obtained 2H-TaS 2 after electrochemical 1000 cycles, annealed 2H-TaS 2 catalyst displayed relatively low overpotential under 0.5 M H 2 SO 4 (160 mV) and 1.0 M KOH (250 mV) solutions, respectively ( Figures 10L and 10M). The subsequently thermal treatment is considered suitable engineering by increasing its porosity and catalytic active sites to accelerate the electrochemical reaction of the prepared TaS 2 . In addition to plasma and thermal annealing, strain (Li et al., 2016b) is another important method that can modulate the defects influencing the HER intrinsic catalytic activity. Locally strained lattices in the zigzag chain reduce the energy required for phase transformation (Voiry et al., 2013b) and thus modulate the hydrogen adsorption and desorption (Li et al., 2016b;Putungan et al., 2015). Voiry et al. reported a stable, strained 1T-WS 2 with high HER activity. As the introduction of tensile strain ($3%) leads to an enhancement in the density of states near the Fermi level, their DG H* approaches zero. In contrast, the compressive strain would cause DG H* on MTMDs (e.g., 1T-MoS 2 and 1T-NbS 2 ) to move further away from zero and decrease the HER activity .

Defect engineering
Another strategy to introduce defect sites in the MTMDs includes the control of synthetic parameters such as precursor and growth conditions. Studies on the correlation between catalytic properties and atomic vacancy have been reported. He et al. showed that the 1T-MoS 2 content can be easily controlled by varying the pyrolysis temperature or Mo/S feeding molar ratios . The disordered stacking of S-Mo-S layers and the abundant defects formed during pyrolysis are synergistically responsible for their high HER activity. However, the content of defective sites in MTMDs must be considered for optimizing the conductivity of the catalysts because the atomic vacancies eventually deteriorate the electrical properties.  (Najafi et al., 2020). Copyright 2020, American Chemical Society.

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Recently, the scope of defects has been expanded to a strategy that can simultaneously improve electrical and catalytic characteristics. Yang et al. utilized a covalently bonded MTMDs by self-intercalation . The metallic 2H and 3R NbS 2 crystals with excess niobium (2H-Nb 1+x S 2 , 3R-Nb 1+x S 2 , and where x is $0.35) are synthesized via the adjustment of CVD synthesis parameters. The prepared Nb 1.35 S 2 materials contained variable thicknesses (2$50 nm) and lateral flake size (0.5$1 mm) determined by the AFM image ( Figure 11A) wherein the crystal structures were identified as excess Nb on both the 2H and 3R-Nb 1.35 S 2 phase from the ADF-STEM images ( Figures 11B and 11C). During the partial occupation of the surplus Nb between 2D metallic NbS 2 layers, the influence of van der Waals forces in the 2D layers declines, which induces great abilities of fast charge transfer and high current capability. Therefore, the 2H-Nb 1.35 S 2 for HER generated an ultrahigh current density of 5,000mAcm À2 at an overpotential of 420 mV (Figures 11D and 11E). The charge density for hydrogen adsorption onto the Nb-terminated surface was theoretically calculated to demonstrate the intrinsic mechanism of Nb 1.35 S 2 catalysts ( Figures 11F and  11G). These results suggest that the localized charge density of the 2H-Nb 1.35 S 2 phase is larger in magnitude than that of the 3R-Nb 1.35 S 2 phase. The Nb self-intercalation in 2H-NbS 2 fabricated by controlling the growth condition of the pressure provides facile defect design to advance HER activity. The chemical doping of MTMDs using various metal dopants such as Co, Ni, Cr, V, or Re can tune the electronic structure . Han et al. presented a one-step CVD method as the facile doping strategy for synthesizing a V single atom-doped metallic 1T WS 2 (V SACs@1T-WS 2 ) monolayer (Han et al., 2021). The V SACs@1T-WS 2 was grown using tungsten trioxide and sulfur while introducing vanadium chloride as the co-precursor ( Figure 11H). Figure 11I shows the atomic structure of high-resolution HAADF-STEM images for V SACs@1T-WS 2 monolayer, which indicates that W atoms are replaced by V atoms. The cross-section HAADF-STEM image in Figure 11J indicates the epitaxial bonding between the V SACs@1T-WS 2 and V 2 O 3 substates in a monolayer, which implies the high-metallic 1T-phase purity of 91%. The catalytic properties of V SACs@1T-WS 2 exhibit lower ᶯ 10 ($185 mV) comparable to 2H-WS 2 counterparts ( Figure 11K). The iScience Review various types of active site in the V single-atomic doped WS 2 catalysts were scrutinized via the theoretical calculation of DG H* . The V-atom sites in the 1T-WS 2 monolayer shows the lowest value of DG H* (0.05 eV) closed to zero, which reveals a superior intrinsic catalytic activity ( Figure 11L). Accordingly, V single-atomic doping considerably affected enhancing the electrocatalytic ability of intrinsic 1T-WS 2 single-layer.

Interfacial engineering
Most catalytic activity of MTMDs for applying electrochemical systems has been measured through catalysts deposited on conductive electrodes (such as carbon paper, glassy carbon, and Ni foam) with binders such as Nafion or polyvinylidene fluoride (PVDF) (Yi et al., 2021;Yu et al., 2014;Zhang et al., 2019c). These additives have critical issues that restrict electrochemical reaction and increases interface resistance because of their inactive and insulating characteristics (Zhang et al., 2019b). Binder-free electrodes have been considered practical catalysts improving adhesion energy between materials and substrates with small interface resistance to overcome this limitation (Hu et al., 2017b;Liu et al., 2018a). Yu et al. found that MTMDs directly grown on a substrate of the equal metal were synthesized by utilizing oriented-solid-phase synthesis (OSPS) to facilitate the mobility of charge injection in the catalysts ( Figure 12A) . They suggested novel systems to fabricate monolith catalyst (MC) based on metallic TaS 2 vertically grown onto the Ta metal (Ta-TaS 2 MC). Figure 12B shows the synthesis of porous Ta-TaS 2 by electrochemical treatment. The cross-section TEM images in Figure 12C clearly reveal an abrupt interface between Ta metal and TaS 2 with intensive covalent bonds. As shown in XRD pattern ( Figure 12D), the structure of the as-synthesized TaS 2 is 3R-phase. Ta-TaS 2 MC accomplished a superior current density of 2,000mAcm À2 with a low overpotential of 398 mV and excellent durability for 200 h toward the commercialization of hydrogen production when the Ta-TaS 2 MC examined HER activity compared with porous Pt foil, Ta foil, and conventional parallel Ta/TaS 2 ( Figures 12E and 12F). These impressive performances of Ta-TaS 2 MC catalysts rose from its features for mechanical strength and electrically near-zero interface resistance. Furthermore, they contributed to the emerging importance of delicate interfacial research for industrializing water electrolyzers. Zhou et al. proposed the method using charge injection between 2H-MoS 2 and T d -WTe 2 via band engineering to improve the interfacial properties ( Figure 12G) . Three devices composed of T d -WTe 2 and 2H-MoS 2 were fabricated corresponding to: (1) Basal plane-MoS 2 contacted WTe 2 , (2) edge-MoS 2 contacted WTe 2 , and (3) basal plane of MoS 2 -WTe 2 heterostructure to verify catalytic activity for the selective windows of MoS 2 ( Figure 12H). The engineered heterostructure (device (iii) in Figure 12H) exhibited the ultimate HER performance (h at 10mA cm À2 (h 10 ) z 150 mV) than WTe 2 contacted MoS 2 (device (i, ii), h 10 z 255 mV), as shown in Figures 12I and 12J. The microdevice of the monolayer MoS 2 demonstrated that the heterojunction between MoS 2 and metallic WTe 2 affect the enhancement of the catalytic characteristic attributed to efficient charge injection.

Other approaches of structural engineering
A beneficial structural design such as alloy (Huang et al., 2019a;Kwak et al., 2020;Kwon et al., 2021) and hybrid structures (Gnanasekar et al., 2020;Zhou et al., 2018b) have been recently dedicated to magnifying the electrochemical capability of MTMDs. Alloying corresponding to the stoichiometry modification of MTMDs compounds mediates conductivity and electronic structure by affecting catalytic activities . Kwak et al. proposed the polytype alloys of Nb 1-x V x Se 2 with the metallic nature as the efficient HER catalysts (Kwak et al., 2022). The Nb 1-x V x Se 2 nanosheets under all composition ranges were grown by a hot-injection colloidal reaction and annealing process ( Figure 13A). The typical SEM images of the Nb 1-x V x Se 2 alloys as a function of composition indicates that the morphologies transformed from nanosheets to thick nanoplates as x increased from 0.2 to 1.0 ( Figure 13B). The Nb 1-x V x Se 2 alloys have a crystal structure of a combination of 2H and 1T phases when x is in the range of 0.1 to 0.3 and have a 1T-phase when x is relatively high ( Figure 13C). The as-prepared Nb 1-x V x Se 2 nanosheets were examined for electrochemical activities; the Nb 0.7 V 3 Se 2 (x = 0.3) showed the ultimate HER performance, which demonstrates that h is the lowest value 236 and 298mVat a current density 10 and 100mAcm À2 , respectively ( Figure 13D). From the DG H* of Nb 1-x V x Se 2 alloys as a function of composition; they confirmed thermoneutral at x = 0.3 and proved the interrelation of the modification for alloy composition with enhanced HER activity ( Figure 13E).
The hybridization between STMDs and MTMDs was reported to enhance fundamental charge transfer efficiency and electrochemical long-term stability . Chen et al. manufactured the MoS 2 nanosheets with metallic 1T-VS 2 (VS 2 @MoS 2 ) via two-step hydrothermal reactions ( Figure 13F) . Representative SEM and TEM images illustrate the distinct formation of MoS 2 nanosheets on the surface of VS 2 nanoflowers ( Figures 13G-13I). The VS 2 @MoS 2 heterostructure with a low h 10 ($177 mV) and Tafel slope ($54.9 mV dec À1 ) demonstrated a higher HER activity than those of the pristine VS 2 and MoS 2 (Figures 13J and 13K). iScience Review Consequently, they suggested VS 2 @MoS 2 heterostructure as electrocatalysts for HER to realize developed electrochemical systems. As a further strategy, composites of MTMDs and conductive materials have been used to improve the catalytic activity and stability. This approach exploits the synergistic benefits of the high catalytic activity of MTMDs, along with the controllable surface, high conductivity, and stable electrochemical properties of the conductive supports. Wang et al. suggested that the HER performance of 1T-VS 2 can be improved by compounding it with V 2 C MXene . When 1T-VS 2 with V 2 C MXene was synthesized via a hydrothermal reaction, the outstanding properties of MXene, such as large surface area and high electrical conductivity, accelerated electron charge transfer, increased the number of exposed HER active sties, and prevented the aggregation of 1T-VS 2 (Figures 13L and 13M). The VS 2 @V 2 C compound exhibits not only a low h 10 value (94 mV) in 0.5 M H 2 SO 4 , but also better activity comparable to Pt/C under a wide range of pH conditions ( Figure 13N). They provided a practical design of compositional structures to achieve superior performance by demonstrating the excellent stability of VS 2 @V 2 C for 200 h ( Figure 13O).

CHALLENGES AND PERSPECTIVES
Researchers continue to focus on developing efficient and sustainable electrocatalysts for promoting the HER to achieve net-zero carbon emission as the world faces an energy crisis. Among some advanced (H) OM images of (i) basal plane exposed, (ii) edge exposed WTe 2 contacted MoS 2 , and (iii) basal plane exposed MoS 2 -WTe 2 heterostructures. (I) Polarization curves of each device.
(J) Comparison of overpotential values in different devices from polarization curves. Reprinted with permission from  iScience Review non-precious-based electrocatalysts, MTMD materials provide the most extensive prospects for material design because of eco-friendly property, ultrahigh conductivity, and tunable and abundant catalytic activity sites. This review summarized extraordinary characteristics and different advances of MTMDs considering synthetic methods and electrochemical catalytic applications (Tables 3 and 4). However, several challenges with current techniques to fulfill the industrialization of MTMD-based electrocatalysts need to be addressed. We hope that the following perspectives will assist the research community on MTMDs materials to surpass current advances in the emerging field of electrocatalysts based on MTMDs and motivate them to develop practical applications ranging from catalytic to electronics and optoelectronics.

Reproducibility and scalable production of MTMDs materials Synthetic possibility of group VIB MTMDs
A familiar synthetic method of group VIB MTMDs is the phase transition from stable 2H to metastable 1T structure by a rather complex post process such as carrier injection (Voiry et al., 2013b) and laser irradiation Reprinted with permission from  iScience Review (Cho et al., 2015). The crystalline quality, stability, and physical properties of group VIB MTMDs can be affected by a variable factor including precursor, synthetic atmosphere, and optimization for post-treatment. Although another synthetic strategy, a solution-based approach can achieve the growth of 1T/2H-group VIB TMDs mixed phase  and fully covered 1T phase via transition metal dopant (Han et al., 2021) or alloying (Kwak et al., 2022); the process parameters are difficult to control. Considering these factors, it is difficult to achieve group VIB MTMDs with reproducible results. Liu et al. recently demonstrated the use of a simple one-step synthetic process to directly grow group VIB MTMDs with the 1T 0 phase via the potassium-assisted CVD (Liu et al., 2018b). This method, however, only produced a few small flakes (generally less than 1 mm), which may limit the fabrication of catalysts. Thus, more approaches with reproducibility should be explored from the perspective of the controllable and scalable production of the stable catalysts of VIB MTMDs with high quality. In recent years, Lai et al. prepared metastable 1T 0 -group VIB MTMDs (including WS 2 , WSe 2 , MoS 2 , MoSe 2 , WS 2x Se 2(1-x) , and MoS 2x Se 2(1-x) ) using a potassium-incorporated metal precursor (Lai et al., 2021). It is worth noting that 1T 0 -group VIB MTMDs are thermally stable up to $120 C or 160 C and have lateral sizes up to a few hundred micrometers. Potassium-containing metal precursors (e.g., K 2 MoO 4 and K 2 WO 4 ) (Lai et al., 2021) or-metal salts (e.g., K 2 C 2 O 4 $H 2 O and K 2 CO 3 ) (Lai et al., 2022) lower the energy required for TMD phase transformation, facilitate electron transfer, and inhibit electron emission. It is expected that these materials will play an important role in solving the large-area fabrication of metastable group VIB MTMDs.

Controllable synthesis of group VB MTMDs
Unlike group VIB TMDs, the synthesis of group VB MTMDs are still in its infancy because of the limitation on the selectivity of the precursor. The low solubility of most transition metal precursors comprising group VB, as summarized in Table 2, interferes with suitable solution-based synthesis. Thus, many researchers deliberately selected the CVD method using the vaporization of a solid precursor as an optimal approach for producing group VB MTMDs materials at the atomic level; this process offers a balance of high quality, high efficiency, controllability, and scalability. Although many breakthroughs have been achieved using the molten salt-assisted metal oxide precursor (Zhou et al., 2018a), there is a concern that the introduction of salt may result in the formation of impurities in the as-synthesized product. Group VB MTMDs grown using these precursors are yet to be truly demonstrated in terms of feasibility as electrocatalysts. Moreover, the commonly used powder vaporization routes using transition metal chlorides with low melting point (e.g., TaCl 5 , VCl 3 , and S powders) are relatively limited in terms of the continuous and constant supply of precursors during the process. These restriction affects the reproducibility and controllability of the composition and thickness because there is a possibility that various intermediate compounds (e.g., V 3 S 4 , VS 2 ) self-intercalated by ordered M atoms within the van der Waals gaps of group VB MTMDs will be produced (Oka et al., 1978;Zhao et al., 2020). More investigation on the role of the various growth parameter and fundamental catalytic properties of fabricated electrocatalysts will be required to facilitate the controllable, scalable, and direct fabrication of group VB MTMDs as electrocatalysts using CVD. For example, Wu et al., discovered that the dangling-bond-free surface of 2D TMDs substrates ensure a minimized diffusion barrier for the precursor atoms in the group VB MTMDs, causing the reactant atoms to migrate to the edge of the growing 2D materials (Wu et al., 2019). They obtained ultrathin group VB MTMDs with thickness as low as 1.0 nm using the 2D substrate effect. Zhao et al. successfully controlled the synthesis of TaS 2 compounds via self-intercalation method by adjusting the Ta/S ratio . DFT calculations were performed to evaluate the thermodynamic stabilities of the intercalated phases. It was found that stoichiometric H-phase TaS 2 was formed only under S-rich conditions, whereas at higher Ta:S flux ratios, various Ta-intercalated Ta x S y compounds attained a thermodynamically stable state. In the far future, compared to the solution-based approach, their high processing cost will be the most important issue that would need to be addressed in terms of economic viability with regard to precursors, facilities, and substrates in future developments.

Durability of MTMDs-based electrocatalysts
The instability of MTMDs that leads to decreased catalytic performance, operational stability, and lifetime of MTMD-based electrocatalysts remains a crucial issue. The instability is primarily attributed to a chemical reaction with H 2 O/O 2 in the ambient environment (Voiry et al., 2013a) and mechanical peeling from the electrode during operation. In general, one is the encapsulation using h-BN (Lee et al., 2015), polymer (N'diaye et al., 2012), and oxide compounds (Wood et al., 2014)  iScience Review not only avoid ingredients from detaching from the electrode to the release H 2 molecules  but also enable fast charge transfer induced by the strong interlayer interaction between the substrate and the catalysts (Kim et al., 2016). Thus, new synthetic strategies are required to directly fabricate the MTMDs-base electrocatalysts on highly conductive substrates without reacting to the electrolyte.

Optimization for the catalytic performance of MTMDs
State-of-the-art HER electrocatalysts fabricated by MTMDs in laboratory-scale systems are summarized in Tables 3 and 4. There are clear pathways forward to enable efficient electrocatalysts, as demonstrated by group VIB STMDs-based electrocatalysts (Li et al., 2016b;Ye et al., 2016). Likewise, the rational design for the best performance of MTMDs materials could be a combination of edge, defect, and interfacial engineering. Recent progress in engineered pure MTMDs shows that these electrocatalysts exhibit superior catalytic performance and stability compared with those of STMDs and other non-precious metal electrocatalysts (such as transition metal-phosphides (Popczun et al., 2013), carbides (Dong et al., 2018), and nitrides ). However, they still suffer from lower performance in comparison to Pt-based groups. One of the key strategies for the future development of MTMDs is the design of the crystal structure because there is a close correlation between the electronic structure, stability, and efficiency. In this regard, the search for new ternary MTMDs (Rezaie et al., 2021), alloying with metals , doped-MTMDs Huang et al., 2019b), and composites based on MTMD and other TM-based materials (Wei et al., 2022) should be attempted. In particular, this method can be a major solution for MTMDs with a relatively lower performance in alkaline solutions. For instance, MTMDs with the incorporation of transition metals, such as Ni-Co-based metallic MoS 2  and NiO-1T MoS 2 (Huang et al., 2019b) or MoS 2 /MXene/CNT ternary composites (Wei et al., 2022), exhibit superior HER activity. Furthermore, MTMDs are expected to be useful as co-catalysts for solar-driven water splitting. Although there are still few reports on the same, the high electrical conductivity and many active sites of MTMDs can indirectly affect the performance of the light absorber by reducing the recombination rate of electron-hole pairs, as well as the HER activity . Recently, heterostructures such as 1T-MoS 2 /g-CN (Xu et al., 2019), 1T-MoS 2 /p-Si (Ding et al., 2014), VS 2 /g-C 3 N 4 (Shao et al., 2018), and NbS 2 /p-Si (Gnanasekar et al., 2019) with an appropriate interface to reduce the charge transfer resistance have been reported (Table 5).

DECLARATION OF INTERESTS
The authors declare no competing interests.